(a) Field of the Invention
This invention relates to automatic welding method and apparatus, and more particularly to a method and apparatus for fabricating pipe centralizers or the like.
(B) Description of the Prior Art
One of the final steps in completion of an oil and gas well is running and cementing the casing. The casing is first run into the hole, and then a cement slurry is forced through the casing to displace drilling mud from the borehole. The cement is then allowed to harden in the annular space between the casing and the borehole, to thereby prevent the movement of fluids between formations. Cementing also serves to support, and prevent corrosion of, the casing.
In order to obtain effective cementing, it is necessary that the casing be centred within the hole so that the cement slurry may completely surround the casing. To achieve such centering, devices known as centralizers are attached at intervals to the casing as it is run.
A centralizer is comprised generally of a pair of end collars adapted to be placed around the casing, and a plurality of bows connecting the collars. The collars are either hinged rings which may be latched about the casing or solid rings which may be slipped over the casing. The internal diameter of the collars is substantially the same as outside diameter of the casing with which they are to be used, and range in size from 23/8 to 60 inches. The bows may be either of spring steel or solid bars. The number of bows in any centralizer depends upon the size of the end collar, with larger diameter centralizers usually containing more bows.
Heretofore, centralizers have been fabricated manually. The initial steps of centralizer fabrication are performed by a welder's helper. The helper first places the end collars upon a work piece holder which usually takes the form of a horizontal metal pipe having marks thereon to enable the helper to space the end collars the proper distance apart. The helper then places a bow upon the collars such that each end thereof is in contact with one of the collars, whereupon he tack welds the ends to the collar. He then rotates the collars with the bow tacked thereto a distance determined by the number of bows to be contained by the finished centralizer and tacks another bow to the collars. The operator continues the process until all of the bows have been tacked to the collars, whereupon he delivers the partially fabricated centralizer to a welder, who lap welds the ends of each bow to the collars to which they have been tacked.
This method of fabricating centralizers is both expensive and inefficient. It is expensive in that it takes a substantial amount of labor to fabricate each centralizer. The welding steps must be performed by welders of at least some experience, and who must therefore be paid a rather high wage. It is inefficient in that it is slow and the output of centralizers produced thereby is limited. It is also inefficient in that it diverts welders from more productive jobs within the shop.